1. Field of the Invention
The present invention generally relates to a magnetic recording medium, and more particularly to a recording medium utilizing selective growth of a ferromagnetic material such as chromium dioxide (CrO.sub.2).
2. Description of the Related Art
Conventional hard disk drive storage media includes a thin (typically 20-30 nm), continuous magnetic film, such as CoPtCr alloy, deposited on a rigid substrate. The commonly-used substrates for thin film disks are Al--Mg alloy plated with A NiP layer, or glass. The data is represented by a bit of a storage element in the media, determined by the orientation of the longitudinal magnetization in a region of approximately 4.0.times.0.15 mm.sup.2 (for 1 Gbit/in.sup.2 areal density). Each of these bits consists of polycrystalline grains of average size 15-20 nm. The grains are partially isolated in order to break the magnetic exchange coupling between them. This is often accomplished by depositing the magnetic layer on top of a Cr underlayer with a columnar structure consisting of voided grain boundaries. The magnetic layer takes on the morphology of the underlayer, thereby resulting in the decoupling of the magnetic grains.
In other cases, segregation of Cr to form non-magnetic Cr-rich grain boundaries also helps in magnetically decoupling the grains. The grains in the magnetic films have a broad distribution in the size and shape, with random crystalline orientations.
For high density longitudinal recording, it is necessary to make the recording medium thin and highly coercive. This is because the demagnetization in the medium not only decreases the remanent magnetization, but also rotates the magnetization vector to establish a circular magnetization mode (Magnetic Recording, Vol. 1: Technology, by C. D. Mee and E. D. Daniel, McGraw Hill Book Company, New York, 1987). Reducing the film thickness prevents the circular magnetization from establishing.
Furthermore, to increase the storage density capacity of the media by decreasing the bit cell, the size of the individual grains must be decreased to keep the number of grains in a bit cell constant at approximately 500-1000 grains.
Without this scaling, the magnetic signal-to-noise ratio (SNR) would increase substantially due to the random orientation of the magnetic easy axis of each grain and their size distribution. Thus, a factor of two scaling leads to a factor of four improvement in areal density, but simultaneously requires a factor of eight reduction in volume resulting in a similar decrease in the magnetic energy stored per grain.
As the grains become very small and weakly coupled to the neighboring grains, the magnetization energy becomes comparable to the thermal energy resulting in loss of the written data due to random thermal switching of the magnetization direction. This thermal switching limits the achievable areal density that the recording medium can support and is usually referred to as the "superparamagnetic limit" (B. Cullity, Introduction to Magnetic Materials, Addison-Wesley Publishing, Reading, Mass. (1972), Chapter 11.6).
It has been estimated that the superparamagnetism will limit the storage density for the currently used (e.g., conventional) magnetic media at about 40 Gbit/in.sup.2. Other limiting factors for recording density are the transition width between two recorded bits determined by the width of the domain walls, side tracks caused by fringing fields of the write head, and density loss in the media required to allow for tracking.
Alternative recording approaches are being considered to further increase the recording density of the media. One approach uses perpendicular recording where the media is magnetized perpendicular to the surface of the disk using materials, such as CoCr alloys, which possess a very strong vertical magnetocrystalline anisotropy. A medium with perpendicular magnetization can in principle exhibit sharp transitions between regions of opposite magnetization because the internal demagnetizing field approach zero near the transition. If the entire film thickness can be magnetized in the perpendicular direction, then high density recording is possible for thick films (e.g., 0.1-1 mm, as compared to thickness of 20-30 nm for longitudinal recording). Thus, since the thickness of the media for perpendicular recording is larger than that for longitudinal recording, the volume per magnetic grain can be correspondingly larger. It has been estimated that a factor of 2-4 increase in areal density may be possible with perpendicular recording before the superparamagnetic limit is reached.
Much larger improvements in areal density are expected, both for longitudinal and perpendicular recording, if every bit cell is isolated and corresponds to a single magnetic domain consisting of a number of polycrystalline grains, as shown in FIG. 1. Specifically, FIG. 1 illustrates a magnetic disk 10 having longitudinal CrO.sub.2 magnetic bits 11.
As shown, in the ultimate limit, the magnetic media includes discrete, single-domain magnetic elements uniformly distributed on the surface of the nonmagnetic disk. Each magnetic element has a uniform, well-defined shape at a specific location, with two stable magnetization directions of equal magnitude representing the binary bits. The size and shape of the elements determine the magnetic field needed to switch the magnetization direction. Since the writing process in a single domain media requires flipping (e.g., switching) the magnetization direction of a discrete bit, it results in much lower noise and lower error rate with correspondingly high density. Furthermore, the transition noise between bits is very small and has significantly reduced cross talk. Tracking also is considerably simplified because of the isolation of each bit.
However, unlike conventional disks, fabrication of one bit per cell media requires photolithographic definition of each grain. More particularly, for single domain elements which have a 50-100 nm size, nanofabrication techniques are essential (e.g., see P. R. Krauss and S Y. Chou, J. Vac. Sci. Technol. B 13, 2850 (1995); S. Y. Chou and P. R. Krauss, J. Magnetism and Magnetic Materials, 155, 151 (1996); S. Y. Chou, Proc. IEEE 85, 652 (1997)). Nanofabrication techniques have been used to produce ultra-high density storage elements based on single domain elements with a storage density of about 65 Gbit/in.sup.2.
However, nanolithographic techniques, such as x-ray and e-beam lithography, are very slow and prohibitively expensive processes which preclude their usage for mass manufacturing of magnetic disks.
To meet the high throughput and low cost requirement for fabricating patterned magnetic nanostructures, Chou et al. (P. R. Krauss and S. Y. Chou, Appl. Phys. Lett., 71, 3174 (1997)) have demonstrated a nanoimprint lithography technique for producing sub-10 nm features by replication.
The process involves creating a resist relief pattern by deforming the resist's physical shape with embossing. In one of the imprint methods used by Chou et al., the resist is a thermoplastic polymer which is heated during the imprint to soften the polymer relative to the mold. The polymer becomes a viscous liquid and can flow above the glass-transition temperature of the polymer, and therefore can be deformed readily to the shape of the mold. Nano-compact disks with 400 Gbit/in.sup.2 storage density have been fabricated using this technique. Similar recording densities are expected for magnetic storage media using single domain magnetic structures patterned using nanoimprint lithography.
Chromium dioxide (CrO.sub.2) is widely used as a particulate magnetic recording medium in tapes (e.g., see D. J. Craik, Magnetic Oxides, John Wiley & Sons (1975), Chapter 12.). The CrO.sub.2 particles are acicular and are comparatively clean and free of dendrites, unlike .gamma.-Fe.sub.2 O.sub.3 particles which also are used as particulate media.
Consequently, the CrO.sub.2 particles are relatively easily dispersed and oriented, and can be used to form magnetic tape which has excellent short wavelength response. The material has a room temperature saturation moment (M.sub.s) of 90-100 emu/g as compared with 74 emu/g for .gamma.-Fe.sub.2 O.sub.3. By varying the additives and processing conditions, CrO.sub.2 particles have been produced having coercivities from less than about 100 Oe to more than about 650 Oe. Because of the higher moment and orientation and greater coercivity, larger output signals at low densities are possible.
Since CrO.sub.2 is a metastable phase, it normally has to be synthesized at high oxygen pressures (500-3000 atmospheres). While this can be quite readily accomplished in the case of bulk synthesis, it has proved to be an impediment for the growth of high quality films. This is because conventional vacuum deposition techniques, such as evaporation and sputtering, usually operate at low pressures.
There have been some reports of CrO.sub.2 films growth at atmospheric pressures by chemical vapor deposition using CrO.sub.3 as a precursor (e.g., see S. Ishibashi, T. Namikawa and M. Satou, Mat. Res. Bull. 14, 51 (1979)). However, reproducible growth of single phase material has been limited to a very narrow window of process conditions. More particularly, it has been suggested that a substrate temperature very close to 390.degree. C. is necessary for single phase growth, and a temperature increase or decrease of even 10.degree. C. results in formation of secondary phases. Additionally, the growth has been limited to single crystal substrates of sapphire and TiO.sub.2 substrates.